1. Field of the Invention
The present invention relates to an epoxy-modified polyimide, and an innovative photosensitive composition and a printed wiring board using the epoxy-modified polyimide and more specifically relates to an epoxy-modified polyimide to be used especially in electronic material field, its production method, as well as a photosensitive composition containing the epoxy-modified polyimide and a photoreaction initiator and excellent in heat resistance, chemical resistance, an insulating property, and flexibility, a material for an electric wiring board using the polyimide, especially, a solder protection film (a solder resist), a coverlay film, and a printed wiring board.
2. Description of the Prior Art
Recently, electronic apparatuses have rapidly been made multifunctional and high-performance and miniaturized and following that, electronic parts are required to be miniaturized and made light weight. For that, as compared with a common rigid printed wiring board as a wiring board to mount electronic parts thereon, a flexible printed wiring board (hereafter referred to as FPC) excellent in flexibility has drawn more attention than ever and the demand has sharply been increased.
Meanwhile, a polymer film, so-called a coverlay film, is stuck to the surface of a FPC for the purpose of protecting the conductor face. A method employed as a method for laminating the coverlay film onto the surface of the conductor face is generally carried out by overlaying the coverlay film processed to have a prescribed shape and bearing an adhesive in one side to a FPC, properly positioning them, and then thermally laminating them each other by pressing or the like. However, the adhesive employed for that case is mainly an epoxy type or acrylic type adhesive or the like and in the case of using such an adhesive, there is a problem that the heat resistance for soldering or the heat resistance of the adhesion strength at a high temperature is low and that the flexibility is inferior and the function of the coverlay film as a base film has not sufficiently been utilized.
Further, in the case a coverlay film is stuck to a FPC using a conventional epoxy or acrylic adhesive before the laminating, it is required for the coverlay film to be processed to form holes and windows positioned at terminal parts of a circuit and junctions of constituent parts. However, not only it is difficult to form holes or the line in a thin coverlay film but also it is required to carry out the positioning conformation of the holes or the like of the coverlay film to the terminal parts of the circuit and junctions of the constituent parts of the FPC almost though a manual work and thus the workability and the positioning precision have been low and the cost has been high.
In order to improve the workability and the positioning precision, a method for forming a protection layer on a conductor face by applying a photosensitive composition and a photosensitive coverlay film (also called as a photosensitive dry film resist) have been developed and the workability and the positioning precision have been improved.
Nevertheless, for the foregoing coverlay film, acrylic type resins have been used and they are insufficient in the heat resistant temperature and brittleness as a film. From the view of improving the heat resistance and the mechanical strength as a film, a photosensitive polyimide has, therefore, been required to be used. Subsequently, there have been developed photosensitive polyimides into which methacryloyl group is introduced with the ester bond linkaged (Japanese examined Patent Publication No. 55-030207, Japanese examined Patent Publication No. 55-041422) and photosensitive polyimides into which amine compounds or diisocyanate compounds having methacryloyl group are introduced in the carboxy group part of the polyamide acid (Japanese unexamined Patent Publication No. 54-145794, Japanese unexamined Patent Publication No. 59-160140, Japanese unexamined Patent Publication No. 03-170547, Japanese unexamined Patent Publication No. 03-186847, Japanese unexamined Patent Publication No. 61-118424).
However, since those photosensitive polyimides are processed to be converted into polyimides by layering them in polyamide acid state on a FPC, exposing and developing the resultant layer, and then carrying out imidation for the layer, the FPC has to be heated to 250xc2x0 C. or higher for the imidation reaction. Further, for some photosensitive polyimides, the acryloyl group has to be removed by heat and during the removal process, there occurs a problem that the film thickness is significantly lessened.
As a photosensitive polyimide which does not require the heat treatment at a high temperature after exposure and development, a method was proposed (Japanese unexamined Patent Publication Number 59-108031) which comprises steps of previously synthesizing a diamine having a carbonxe2x80x94carbon double bond, causing reaction of the diamine with an acid dianhydride to obtain a polyamide acid, and causing imidation of the resultant polyamide acid to use the obtained compound as a photosensitive resin. In this case, since the imidation is carried out before the exposure and development, it is no need to carry out heat treatment at a high after the exposure and development.
However, by the method proposed in the Japanese unexamined Patent Publication Number 59-108031, it is difficult to produce the diamine compound having a carbonxe2x80x94carbon double bond at high yield. The reason why is that when a dinitro compound having a carbonxe2x80x94carbon double bond, from which the diamine compound is generally produced by reduction, is reduced, the carbonxe2x80x94carbon double bond also tends to be reduced. Subsequently, the problem caused is that the yield is lowered and therefore the diamine compound having a carbonxe2x80x94carbon double bond inevitably becomes expensive.
In addition to that, the formation process of a solder protection film (e.g. a solder resist) is almost essential one in the printed wiring board manufacture. As a solder resist for a printed wiring board for industrial use, there conventionally used are thermosetting type solder resist compositions of alkyd/melamine resins, epoxy/melamine resin, two-part epoxy resins, and the likes, and these thermosetting solder resists are applied to the surface of a printed wiring board by printing or other methods and then hardened by heating to form a protection film.
However, those conventionally used solder resists require a relatively long time and high temperature for hardening by heating after application to be lower the productivity and cause a problem that substrate warping and shrinking easily occurs owing to the high heat hardening temperature. For the purpose of solving those problems, ultraviolet ray (UV)-curable solder resists used for printed wiring boards for consumer use have been studied and, in spite of that, found the properties are not satisfactory. Moreover, for printed wiring boards for industrial use, the conductor thickness of such as a copper foil has to be thicker, about 50 to 70 xcexcm, as comparison consumer-use one having thickness of at most 60 xcexcm, usually 40 xcexcm, and a industrial-use solder resist has to be applied to be thick. On the contrary, a UV-curable solder resist for consumer use has a problem in the hardenability in case of a thick film and has to be thin. Nevertheless, if the thickness of the solder resist is made thin, there occurs a problem that the insulation property and the heat resistance for soldering which are required for printed wiring boards for industrial use become unsatisfactory and the reliability is deteriorated.
Furthermore, in the case of a flexible printed wiring board not only for industrial use but also for consumer use, additionally to the foregoing reliability, excellent film-formability and flexibility are required and hence conventional thermosetting and UV-curable solder resists are unsatisfactory. Subsequently, a photosensitive solder resist is desired o be developed for the use of a printed wiring board and a flexible printed wiring board for industrial use.
Taking the foregoing circumstances into consideration, the present invention has been developed to provide a photosensitive composition capable of forming a protection film excellent in electric insulation property, heat resistance for soldering, film-formability, flexibility, and chemical resistance and further to provide a coverlay film, a solder resist, and a printed wiring board using the composition and to provide their production methods.
Hence investigation has been performed for the purpose of solving the foregoing conventional problems and providing an economical photosensitive polyimide having sufficiently high mechanical strength and excellent in heat resistance and further in the workability, adhesion strength and especially in low moisture-adsorption property.
More specifically, the present invention provides a method for freely producing a polyimide having a variety of functional groups not limited to a carbonxe2x80x94carbon double bond by modifying polyimide having hydroxy groups and carboxy groups with an epoxy compound and the production method has been found possible to solve the above described objects. That is, an epoxy-modified polyimide of the present invention can be produced by a production method wherein a polyimide having carboxy groups or hydroxy groups is obtained and then the polyimide is reacted with an epoxy compound having two or more epoxy groups or an epoxy compound having an epoxy group together with a carbonxe2x80x94carbon double bond or an epoxy group and a carbonxe2x80x94carbon triple bond.
An epoxy-modified polyimide comprises at least 1% by weight of a structural unit represented by the general formula (1): 
wherein R1 is a tetravalent organic group;
R3 is a divalent organic group; and
R2 is a monovalent organic group containing one to four organic groups selected from the group consisting of: 
wherein R4 is a monovalent organic group having at least one selected from the group consisting of an epoxy group, a carbonxe2x80x94carbon double bond, and a carbonxe2x80x94carbon triple bond;
m is an integer of one or more; and
n is an integer of 0 or more.
In the general formula (1), R1 may be at least one tetravalent organic group containing one to three aromatic rings or alicyclic group.
In the general formula (1), R3 may be a diamine residue containing at least one of a carboxy group and a hydroxy group.
In the general formula (1), R1 may be a residue of acid dianhydride containing at least 10 mol % of acid dianhydride selected from the group (I): 
wherein R5 represents xe2x80x94C6H4xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94, and a single bond; R6 is a divalent organic group selected from the group (II): 
wherein p is an integer from 1 to 20,
wherein T is one of H, F, Cl, Br, I, and MeOxe2x80x94, and an alkyl group having 1 to 20 carbons; and
R7 is one of xe2x80x94Oxe2x80x94 and xe2x80x94COOxe2x80x94.
In the general formula (1), R2 may be a diamine residue selected from the group (III): 
wherein q and s are an integer from 1 to 3; r is an integer from 1 to 4; and R8 is a divalent organic group selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, and xe2x80x94Oxe2x80x94CH2xe2x80x94C(CH3)2xe2x80x94CH2xe2x80x94Oxe2x80x94.
In the general formula (1), R3 may be a diamine residue selected form the group (IV): 
wherein T is one of H, F, Cl, Br, I, CH3Oxe2x80x94, and an alkyl group having 1 to 20 carbons;
A is one of xe2x80x94C(xe2x95x90O)xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94(CH2)mxe2x80x94, xe2x80x94NHCOxe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94Oxe2x80x94CH2xe2x80x94C(CH3)2xe2x80x94CH2xe2x80x94Oxe2x80x94, and a bond;
R9 is one of xe2x80x94Oxe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94O(Oxe2x95x90)Cxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94C(xe2x95x90O)xe2x80x94, xe2x80x94Sxe2x80x94,xe2x80x94C(CH3)2xe2x80x94, and a bond;
k is an integer from 0 to 4;
t is an integer from 1 to 4; and
j is an integer from 1 to 20.
A method of producing an epoxy-modified polyimide comprises: reacting polyimide represented by the general formula (2): 
wherein R10 is a tetravalent organic group; R11 is a divalent organic group containing at least one of a carboxy group and a hydroxy group; R12 is a divalent organic group; m is an integer of one or more; and n is an integer of 0 or more, with a compound selected from the group consisting of an epoxy compound containing at least two epoxy groups, a compound containing an epoxy group and a carbonxe2x80x94carbon double bond, and a compound containing an epoxy group and a carbonxe2x80x94carbon triple bond.
A glass transition temperature of the polyimide represented by the general formula (2) is 350xc2x0 C. or less.
The method of producing an epoxy-modified polyimide further comprises: preparing polyamide acid by reacting diamine with acid dianhydride in an organic solvent when the polyimide represented by the general formula (2) is synthesized; and subsequently forming polyimide by heat drying the organic solvent solution of polyamide acid under a reduced pressure.
The drying temperature may be in a range form 80xc2x0 C. to 400xc2x0 C.
The drying pressure may be in a range from 0.09 to 0.0001 MPa.
A photosensitive composition comprises:
(A) an epoxy modified polyimide having a structural unit represented by the general formula (1): 
wherein R1 is a tetravalent organic group; R3 is a divalent organic group; and R2 is a divalent organic group containing 1 to 4 organic groups selected from the group consisting of: 
wherein R4 is a monovalent organic group containing at least one selected from the group consisting of an epoxy group, a carbonxe2x80x94carbon double bond, and a carbonxe2x80x94carbon triple bond;
m is an integer of 1 or more; and
n is an integer of 0 or more, and
(B) at least one of a photoreaction initiator and a sensitizer.
In another embodiment, a photosensitive composition comprises:
(A) an epoxy modified polyimide having a structural unit represented by at least the general formula (1): 
wherein R1 is a tetravalent organic group; R3 is a divalent organic group; and R2 is a divalent organic group containing 1 to 4 organic groups selected from the group consisting of: 
wherein R4 is a monovalent organic group containing at least one selected from the group consisting of an epoxy group, a carbonxe2x80x94carbon double bond, and a carbonxe2x80x94carbon triple bond;
m is an integer of 1 or more; and
n is an integer of 0 or more,
(B) at least one of a photoreaction initiator and a sensitizer, and
(C) one of a reactive monomer or an oligomer containing a reactive carbonxe2x80x94carbon unsaturated bond.
A coverlay film according to the present invention consists essentially of the above-described photosensitive composition.
A solder resist according to the present invention consists essentially of the above-described photosensitive composition.
A printed wiring board according to the present invention comprises a solder resist made of the above-described photosensitive composition.
The printed wiring board according to the present invention includes at least one of a solder plating layer and a gold plating layer on a portion other than the coverlay film or the solder resist.
A method for producing a printed wiring board comprises: forming a protective layer by applying the above-described photosensitive composition onto a printed wiring board; and photo-curing the board.
A method of producing a printed wiring board comprises: forming a protective layer from a solder resist by applying the above-described photosensitive composition onto a printed wiring board and photo-curing the board; and applying solder plate or gold plate onto the portion other than the protective layer.
An epoxy-modified polyimide of the present invention provides an economical photosensitive composition excellent in heat resistance, adhesion strength, and chemical resistance and is useful for electronic material field, especially as an electric wiring board material. A production method of the present invention is easier than a conventional photosensitive polyimide production method and capable of introducing a variety of functional groups, increasing the yield, and improving the productivity. Further, a photosensitive composition containing an epoxy-modified polyimide of the present invention is preferably used for a coverlay film and a solder resist and a printed wiring board using thereof is excellent in heat resistance for soldering, chemical resistance, electric insulation property, and flexibility and a printed wiring board with high reliability required in the industrial electronic field can be manufactured.